Now, because side DA is equal to side BA,
and side CA is common to triangles CAD, CAB,
the two sides CA, AD are equal to the two sides CA, AB respectively;
and we have shown that the base CD is equal to the base CB;
therefore angle CAD is equal to angle CAB. S.S.S.

But angle CAD is a right angle; Construction
therefore angle CAB is also a right angle. Axiom 1